Electron system for convergence of electrons from photocathode having curvature in asingle plane



R. H. CLAYTON July 2, 1968 ELECTRON SYSTEM FOR CONVERGENCE OF ELECTRONS FROM PHOTOCATHODE HAVING CURVATURE IN A SINGLE PLANE Filed July 28, 1965 2 Sheets-Sheet 1 INVENTOR ROBERT H. CLAYTON BY Z/MQ, w 4 M2 ATTORNEYS y 1968 R. H. CLAYTON 3,391,295

ELECTRON SYSTEM FOR CONVERGENCE OF ELECTRONS FROM PHOTOCATHODE HAVING CURVATURE IN A SINGLE PLANE Filed July 28, 1965 2 Sheets-Sheet z I 40 g. 49 4? j m 6 4 50 6 48 INVENTOR ROBERT H. CLAYTON ATTORNEYS United States Patent 3,391,295 ELECTRON SYSTEM FOR CONVERGENCE OF ELECTRONS FROM PHOTOCATHODE HAV- ING CURVATURE IN A SINGLE PLANE Robert H. Clayton, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Filed July 28, 1965, Ser. No. 475,465 11 Claims. (Cl. 313-65) ABSTRACT OF THE DISCLOSURE An image tube is provided with an electron lens at the cross-over point of the total electron beam so that incremental electron rays are converged and focused for optimum spot resolution without affecting the magnification of the total beam. A photocathode and accelerating mesh and a flat longitudinal drift electrode enclosing the beam provide convergence of the total beam. Deflection plates, a scanning aperture and an electron multiplier are employed in an image dissector configuration.

This invention relates generally to phototubes and more particularly to an image dissector tube.

A conventional image dissector tube comprises a photocathode for emitting an electron beam in response to incident radiation, the beam being area-modulated to provide an electron image in response to an incident radiation image. In all electrostatic versions, the beam is accelerated by an accelerating electrostatic field and electrostatically focused, and is deflected over a defining aperture. The electrons which pass through the aperture being received by target electrode means, typically an electron multiplier, thereby providing a time-based video output signal. In such conventional image dissector tubes, a distributed electrostatic lens is provided which causes the beam to converge to a crossover point and thereafter diverge. The initial electron emission from the surface of the photocathode includes tangential components and in conventional electrostatically focused tubes, the distributed focusing field provides convergence of these tangential components over the entire trajectory of the beam.

The electrons emitted from an incremental spot on the surface of the photocathode can be considered to be a ray or bundle of electrons and thus in conventional electrostatically focused tubes, both the entire beam and the individual incremental rays or budles forming the beam are focused by the distributed electrostatic lens system. In such tubes, however, the condition of optimum focus of the electron image is established by the location of the minimum circle of confusion of the reconverged incremental rays or bundles of electrons, i.e., the minimum sized spot provided on the image surface in response to emission from an incremental spot on the photocathode surface. In the reconvergence of the incremental rays or electron bundles, the distributed electrostatic lens system has its principal plane somewhere between the cathode and anode, generally approximately midway therebetween. Thus, in order to obtain the minimum circle of confusion of the reconverged incremental rays or electron bundles at the image surface, i.e., optimum spot resolution, an image distance is provided which is large compared with the object distance, which in turn results in relatively large magnification and poor resolution. In most optical systems, reduction in the magnification ratio results in improvement in resolution. However, in such conventional tubes, the distributed electrostatic lens system provides convergence of the incremental rays or electron bundles at a minimum circle of confusion point spaced from the point of convergence and divergence of the total beam,

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i.e., the cross-over, and thus an effort to reduce the magnification results also in increasing the spot size of the incremental rays or electron bundles thus deteriorating the resolution.

It will thus be observed that in conventional electrostatically focused image tubes, there have been two conflicting requirements, i.e., magnification of the total image and spot resolution, optimum spot resolution, however, determining overall magnification. It is therefore desirable to provide a phototube, i.e., a multiplier phototube, image dissector or image converter, in which magnification of the image is determined independently of spot resolution.

In addition to the foregoing, there are applications for a camera tube which scans in only one direction, relative motion in the other direction between the camera tube and the optical image being viewed providing frame scanning. It is also at times desirable to provide such a camera tube which will accommodate an extended image in the scanning direction, i.e., a relatively long line. It may further be desirable in applications for such tubes to employ a reflection optical system, i.e. folded optics by reason of the greater light gathering power and reduced weight of such systems with respect to reflective optical systems of comparable performance. In the case of such folded optical systems, it is further desirable to limit the volume of the camera tube and especially the head-on cross-sectional area. In a simple folded optical system, the camera tube obstructs a part of the incident light, and thus a tube having a smaller head-on cross-sectional area, in a plane perpendicular to its axis, increases the effective aperture of the optical system. While double folded optical systems, such as the Cassegrainian system can overcome obstruction by the camera tube, in many applications where a wide field of view is desired in the optical system, the focal length is too short in such a double folded system. It is therefore desirable to provide a flattened camera tube which will accordingly receive more light from a simple folded optical system.

It is further desirable in camera tube applications of the type discussed above to employ a multiplier-type tube, such tubes having a dynamic range exceeding other types of camera tubes, i.e., its signal output is linear over a more extended range as a function of the incident light intensity. Of all presently existing camera tubes, the image dissector with photomultiplier output is the only one in which the detectivity is limited only by the photoelectron statistics; the capability of an image dissector to detect emitted electrons terminates only when the number of electrons emitted per band width increment becomes statistically indistinct. Furthermore, an image dissector is not an integrating device such as the image orthicon and vidicon tubes, and thus the absence of integration permits changing of the scanning rate at will.

It is therefore desirable to provide an image dissector tube of the photomultiplier type adapted for line scanning and having a fiat configuration to provide minimum headon cross-sectional area, and it is further desirable that such a tube be capable of minimum magnification with optimum resolution, the spot resolution being independent of the magnification is aforesaid.

It is accordingly an object of the invention to provide an improved phototube.

Another object of the invention is to provide an improved phototube in which spot resolution is independent of magnification.

A further object of the invention is to provide a line scanning image dissector tube.

Yet another object of the invention is to provide a line-scanning image dissector tube having minimum headon cross-sectional area and spot resolution independent of magnification.

In accordance with the broader aspects .of the invention, a phototube is provided comprising photocathode means for emitting an electron beam in response to incident radiation. First electrode means is provided spaced from the photocathode means for accelerating the elec trons of the beam and second electrode means is provided cooperating with the first electrode means for providing a field-free electron drift space. In accordance with the invention, the photocathode means and the accelerating electrode means are cooperatively arranged to provide a field for converging the beam to a cross-over point spaced from the drift space, the beam thereafter diverging from the cross-over point. Electron lens means is provided adjacent the cross-over point for reconverging the electrons of the beam having initial velocities tangential to the surface of the photocathode means. The electron lens means adjacent the cross-over point of the total or gross beam thus focuses the incremental electron rays or bundles to provide optimum spot resolution independently of magnification of the total beam.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view of a line scan image dissector tube incorporating the invention;

FIG. 2 is a schematic cross-sectional view of the tube .of FIG. 1 taken along the line 22 thereof;

FIG. 3 is a diagram showing beam focusing in conventional prior image tubes and useful in explaining the invention; and

FIGS. 4 and 5 are diagrams showing the focusing system of the present invention and useful in explaining the same.

Referring now briefly to FIG. 3, a photocathode is shown emitting an electron beam 11 in response to incident radiation, the beam 11 being accelerated by conventional accelerating electrodes, shown schematically at 12 as being a single electrode, but commonly comprising several electrodes coupled to different potentials, provide a distributed electrostatic focusing field which can be said to form an electrostatic lens shown in dashed lines 13, which converges the total or gross electron beam 11 to a cross-over point 14, the total beam thereafter diverging as at 15. Considering now a single incremental illuminated spot 16 on the surface of the photocathode 10, the electrons emitted from the spot 16 can be considered to be a ray or bundle of electrons 17, these rays or bundles of electrons forming the total electron beam 11. Electrons are emitted from the spot 16 ,on the photocathode 10 in something approaching a cosine squared density distribution, and thus there are electrons which are emitted tangentially from the spot 16. The distributed electrostatic lens system 12 of the conventional tube reconverges these tangential electrons of each incremental ray, thus converging the incremental ray or bundle of electrons 17 to a circle of minimum confusion 18 after which the ray diverges again, as at 19. It is thus seen that the cross-over point or point of minimum circle of confusion 18 of the incremental ray or bundle of electrons 17, which establishes the condition of optimum focus and thus the image surface 20, is not the same as the cross-over point 14 of the total or gross beam 11. It is further seen that, under this condition, the image distance S is relatively great compared with the object distance the magnification ratio being proportional to S /S It will now be seen that merely moving the image surface 20 toward the photocathode 10, while reducing the magnification, will result in deterioration of the spot focus of the incremental ray 17 since the circle of confusion 18 will become larger. Finally, it will be seen that the magnification and spot resolutions are interdependent, any

I ing both the magnification and the spot resolution.

Referring now to FIGS. 1 and 2 of the drawing, there is shown the flat line scan image dissector tube of the invention, generally indicated at 22, comprising an enclosing envelope 23 having opposite ends 24,25 and a major axis 26. Envelope 23 is formed of suitable insulating material, such as glass, and end 24 is the faceplate of the tube and is transparent to the incident radiation of the wavelength which the tube is intended to detect. Envelope 23 has fiat parallel top and bottom walls 27, 28 and side walls 29, 30 which define a funnel portion 32 and a neck portion 33, the faceplate 24 thus being wider than the end 25. In accordance With the invention, faceplate 24 is formed as a section of a cylinder having its radius of curvature extending from axis or center of curvature 33 which is on the longitudinal axis 26 of the envelope 23 normal to the top and bottom walls 27, 28. Photocathode 10 is deposited in any conventional manner on the inner surface of the faceplate 24 and may, in accordance with conventional practice, have a trans parent conductive coating deposited thereon to which a external lead 34 is coupled.

A tunnel electrode assembly 35 is provided comprising spaced, parallel, flat top and bottom walls 36, 37'respectively parallel with the top and bottom envelope walls 27, 28, and contained within side walls 38, 39 which respectively converge toward envelope end 25. Tunnel electrode 35 has a rear end wall 40 with a central aperture 42 therein symmetrically disposed on the major axis 26 of the envelope 23. A fine mesh conductive screen 44 extends across the front end of the tunnel electrode 35 being connected to the top and bottom walls 36, 37, and the side walls 38 and 39, mesh 44 likewise forming a section of a cylinder having its radius of curvature extending from axis 33. Mesh 44 is thus uniformly equally spaced from faceplate 24 and the photocathode 10. Tunnel electrode 5 has external lead 45 connected thereto, and thus when appropriate potentials are applied to the photocathode lead 34 and the tunnel electrode lead 45, an accelerating field is provided by the mesh 44 to accelerate the electrons emitted by the photocathode 10 in response to incident radiation toward end 25 of envelope 23.

Mesh 44 top and bottom walls 36, 37, side walls 38,- 39 and end wall 40 of tunnel electrode 35, all being connected to external lead 45 and thus to the same source of potential, provide a field-free drift space for the electrons emitted by photocathode 10 and initially accelerated by the accelerating field provided between the mesh 44 and the photocathode 10. It will now be seen that by reason of the curvature of photocathode 10 and mesh 44 about axis 33, the total or gross electron beam 11 emitted by the photocathode 10 will be converged to axis 33 as the cross-over point, the beam diverging again beyond axis 33 as shown in FIG. 4 at 15. It will further be seen that by reason of the forming of the photocathode 10 and mesh 44 as segments of concentric cylinders, the electron beam 11 is emitted in planes parallel to the top and bottom walls 27, 28 and converged in those parallel planes.

An einzel lens 46 is provided at the axis 33, einzel lens 46 comprising plates 40, 47 and 48 having apertures 42, 49 and 50 therein symmetrical about the major axis 26 of the envelope 23, and the end wall 40 of the tunnel electrode 35. The central apertured plate 47 of the einzel lens 46 is connected to external lead 52 while plates 40, 48 are connected to external lead 45.

Plate electrode 53 is provided having a beam-defining aperture 54 therein symmetrically disposed on axis 26 of the envelope 23, plate 53 being connected to external lead 55. A conventional photomultiplier 56 is provided between the apertured plate 53 and the end wall 25 of envelope 23, photomultiplier 56 having its initial stage 57 facing aperture 54 and having target electrode 58 associated with the final multiplier stage 59. The successive stages of the photomultiplier 56 are coupled to leads 60 which are adapted to be connected to progressively higher voltages by an internal or external voltage divider in accordance with conventional practice. The target or output electrode 58 is connected to external lead 62, which in turn is adapted to be connected to a video signal output circuit in accordance with conventional practice.

A pair of line deflecting electrodes 63, 64 are positioned between the einzel lens 46 and the apertured plate member 53 and are respectively connected to external leads 65, 66 which are adapted to be connected to a suitable source of deflection voltage in accordance with conventional practice. Line deflection electrodes 63, 64 deflect the divergent beam 15 in planes parallel with the top and bottom wall 27, 28 of envelope 23 across the aperture 54 in the plate member 53 with the electron beam passing through the aperture 54 being multiplied by the photomultiplier 56 and generating a time-based video signal, as is well known to those skilled in the art. Line deflection plates 63, 64, thus provide line scanning in the direction shown by the arrows 67 in FIG. 1 and it will be understood that frame scanning is provided by relative motion of the light image across the tube 22 in a direction at right angles to the direction 67 of line scanning, as shown by the arrow 68 in FIG. 2.

Referring now again to FIG. 4, while the arcuate electrostatic field provided between mesh 44 and photocathode converges the total or gross electron beam 11 to form a cross-over at axis 33, which is the coincident center of curvature of photocathode 10 and mesh 44, this field does not reconverge electrons with initial velocities tangential to the surface of photocathode 10. Thus, even though the main beam 11 is converged at axis 33, the incremental rays or bundles of electrons 17 emitted from incremental illuminated spot 16 on the surface of the photocathode 10 continue to diverge, as shown in FIG. 4. Thus, unless means are provided for reconverging the incremental rays or bundles of electrons forming the main beam 11, 15, the resulting spot 69 on any chosen image surface 20 would inherently be larger than the spot 16 on the photocathode 10, thus providing inherently poor resolution.

Referring now to FIG. 5, since the incremental rays Or bundles of electrons 17 have not been converged by the field established by the mesh 44 and photocathode 10, it becomes possible to locate an electrostatic lens 46 at the crossover point 33 of the main beam 11 for focusing the incremental rays 17. Lens 46, being at the cross-over point 33 of the main beam 11 thus does not aflect the convergence and divergence of the main beam 11, but does reconverge the incremental rays 17 through a minimum circle of confusion spot 70 on the image surface 20. It will be seen that the reconvergent field provided by the electrostatic lens 46 may be suitably adjusted to determine the location of the minimum circle of confusion spot 70 substantially independently of the magnification provided by a field established by mesh 44 and photocathode 10. Thus, if the magnification provided at the point of optimum spot resolution 70 is too great or too small, the magnification can be adjusted to the desired ratio by adjustment of the field provided by mesh 44 and photocathode 10, or, alternatively, if with the desired magnification ratio the spot resolution is not optimum, it can be optimized by adjustment of the focusing field provided by lens 46.

It will be readily seen that other types of electrostatic lenses well known to those skilled in the art may be employed for the ray reconverging lens 46, such as a bipotential lens or a magnetic short lens. It will be readily seen that, in the case of an image dissector, the apertured plate 53 is preferably positioned immediately adjacent intersection of the image surface 20 with the major axis 26 of the envelope 23.

It will now be seen that the geometry of the mesht44 and photocathode 10 in conjunction with thelocation of the image surface 20, i.e., aperture plate 53, determines the image focus or divergence, i.e., the magnification, while spot resolution of the incremental rays 17 is provided by the central lens 46 which brings the incremental rays or electron bundles 17 into optimum focus at the image surface 20. Y.

It will be observed, by reference to FllG. 2, that the photocathode 10 is flat along any radius extending from axis 33, whereas it is curved in any plane parallel to-the top and bottom walls 27, 28 of the envelope 23. Thus, an inherent astigmatic condition is provided since the initial divergence due to the tangential electron velocities is reduced to a greater extent by the electron field provided by photocathode 10 and mesh 44 in the planes including the curvature than in planes which do not include the curvature. This condition can be corrected by an astigmatism correcting lens or by providing the center lens 46 with one or more of the elements having an elliptical or rectangular aperture. In the embodiment illustrated in FIGS. 1 and 2, an astigmatism lens 72 is provided comprising electrodes 7 3, 74 located within the tunnel electrode 35 and respectively parallel to the top and bottom walls 36, 37 thereof, electrodes 73, 74 being respectively arcuate with radii of curvature extending from the axis 33. Astigmatism correction electrodes 73, 74 are respectively connected to external leads 75. When connected to appropriate potentials, the electrodes 73, 74 form an astigmatism lens which tends to converge both the main beam 11 and the incremental bundles 17 therein in planes normal to the top and bottom walls 27, 28 of the envelope 23 in the manner of a conventional electrostatic lens.

It will be readily apparent that the invention is not restricted to electrostatic deflection, and that a conventional magnetic deflection yoke may be substituted for the deflection plates 63, 64. It will further be seen that the electrostatic focusing system of the invention is not restricted to an image dissector tube, but also may be employed in a multiplier phototube or image converter tube.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of this invention.

What is claimed is:

1. A phototube comprising: photocathode means at one end of said tube for emitting an electron beam in response to incident radiation; first electrode means spaced from and parallel to said photocathode means for accelerating the electrons of said beam; second electrode means cooperating with said first electrode means and extending axially along and around said beam for providing a field-free electron drift space; said photocathode means and said first electrode means being cooperatively curved to provide a field for converging said beam in one plane to a cross-over point spaced from said drift space, said beam thereafter diverging from said cross-over point; electron lens means positioned at said cross-over point for reconverging and focusing incremental electron rays of said beam onto an image area toward the other end of said tube without affecting the total electron beam and means for converging said beam and incremental rays in a plane normal to said one plane.

2. The tube of claim 1 wherein said first electrode means comprises a conductive screen electrode and said second electrode means comprises a conductive tunnel electrode having parallel top and bottom walls and side walls converging from said photocathode end toward said cross-over point, said screen being connected to the end of said tunnel electrode adjacent said photocathode and closing the same.

3. The tube of claim 1 wherein said photocathode means and first electrode means are respectively curved 7 with r-adii'of curvature extending from said cross-over point.

4. The tube of claim 1 wherein said lens is an einzel lens. r

5. The tube of claim 1 further comprising apertured scanning electrode means adjacent said image area target electrode means adjacent said apertured electrode means, and beam deflection means between said lens means and apertured electrode means for deflecting the beam over the aperture in said apertured electrode means thereby providing an image dissector tube.

6. The tube of claim 1 further comprising an evacuated enclosing envelope having opposite ends, one of said ends being a faceplate transparent to said radiation; said photocathode means being deposited on the inner surface of said faceplate; said first electrode means comprising a conductive screen spaced from said faceplate toward the other end of said envelope; said second electrode means comprising a conductive tunnel electrode having opposite ends with oneend connected to and closed by said screen; said tunnel electrode extending toward said other envelope end and having parallel top and bottom walls and side walls converging from said photocathode end toward said cross-over point; said faceplate and screen being curved about radii of curvature extending from said cross-over point; said cross-over point being spaced from the other end of said tunnel electrode toward said other envelope end; said beam and incremental rays being converged in a plane normal to said top and bottom walls onto said image area; an apertured electrode adjacent said image area; electron multiplier means on the side of the aperture in said apertured member toward said other envelope end; and beam deflection means between said lens means and apertured electrode for deflecting the beam over said aperture thereby providing an image dissector tube.

7. The tube of claim 1 wherein said photocathode means and first electrode means respectively are formed as segments of concentric cylinders having radii of curvature extending from said cross-over point; said crossover point being the axis of said cylinders whereby said convergence and divergence of said beam is essentially in planes normal to said axis.

8. The tube of claim 1 wherein said means for converging said beam and incremental rays in a plane normal to said one plane comprises electrode means in said drift space parallel with said one plane.

9. The tube of claim 1 further comprising an evacuated enclosing envelope having a longitudinal axis, opposite ends and generally flat parallel top and bottom walls, one of said ends being a faceplate transparent to said radiation; said photocathode being deposited on the inner surface of said faceplate; said first electrode means comprising a conductive screen spaced from said faceplate; said faceplate and screen being respectively formed as segments of concentric cylinders having radii of curvature extending from said cross-over point; said cross-over point being the axis of said cylinders whereby said convergence and divergence of said beam is in planes essentially normal to said cylinder axis; said cylinder axis being normal to said top and bottom sides and on said envelope axis; said second electrode means comprising a tunnel electrode having opposite ends and generally fiat top and bottom walls respectively parallel with said envelope top and bottom walls; said screen being connected to and closing one of said tunnel electrode ends; said tunnel electrode extending toward said other envelope end and having an apertured end wall at its other end, the aperture in said end wall being on said envelope axis; said cross-over point being spaced from said apertured end wall toward said other envelope end; said tunnel electrode having side walls converging from said one end to said apertured end wall; an apertured scanning plate adjacent said image area in a plane normal to said envelope axis and to said top and bottom walls; the aperture in said plate being on said envelope axis; electron multiplier means on the side of said lastnamed aperture toward said other envelope end; and beam deflection means between said lens means and said apertured plate for deflecting said diverging beam in directions at right angles to said cylinder axis over said lastnanied aperture thereby providing an image dissector tube with scanning in one direction.

10. The tube of claim 9 wherein siad lens means is an einzel lens.

11. The tube of claim 9 wherein said means for converging said beam and incremental rays comprises a pair of electrodes in said tunnel electrode respectively parallel with said top and bottom walls thereof; said pair of electrodes being respectively arcuate with radii of curavture extending from said cylinder axis.

References Cited UNITED STATES PATENTS 3,082,342 3/1963 Pietri 313102 X 3,109,957 11/1963 McGee et al 313-102 X 2,907,907 10/1959 McNaney 313- FOREIGN PATENTS 964,262 7/1964- Great Britain.

ROBERT SEGAL, Primary Examiner. 

